1. Field of the Invention
The present invention relates to a hot-dip zinc-coated steel sheet, particularly, a galvannealed steel sheet which is suitable for use as an anti-corrosive steel sheet for automobiles and which exhibits an excellent press formability.
2. Description of the Prior Art
When a steel sheet is formed into an automobile car body by press forming it is important to consider the sliding characteristics of the steel sheet with respect to the press die. That is, when the sliding characteristics of the steel sheet deteriorate, the flow of the steel sheet into the press die is restricted, which will lead to a fracture of steel sheet material.
Particularly, when a hot-dip zinc-coated steel sheet has a zinc layer on the surface thereof, the sliding resistance thereof with respect to the die during press forming is larger than that of a cold rolled steel sheet, and the sliding characteristics thereof are relatively poor.
In a galvannealed steel sheet, in order to prevent peel-off of a galvannealed layer due to deformation of the steel sheet resulting from press forming, the degree of galvannealing is restricted to a low level to have the iron content of the galvannealed layer relatively low. Therefore, the sliding characteristics of the galvannealed layer with respect to the press die are negatively affected, and a material fracture often occurs during pressing. To eliminate such a problem, it has been proposed in Japanese Patent Laid-Open No. Hei 3-82746 to plate the galvannealed steel sheet with an alloy layer mainly consisting of Fe (iron) plating and thereby improve the sliding characteristics of the galvannealed layer with respect to the press die. In Japanese Patent Laid-Open No. Hei 3-162492, it has been proposed to coat the galvannealed steel sheet with rust-preventatives or press oil exhibiting an excellent lubrication property and thereby promote flow of the material.
Japanese Patent Laid-Open Hei 1-242765 discloses a steel sheet having a flat top portion which is 30 to 90% of the overall area on the surface thereof. The steel sheet exhibits an excellent press formability, as good as a plated steel sheet which has been subjected to an iron type plating.
However, adjustment of the surface roughness alone is not enough to improve the sliding characteristics of the steel sheet. Fe type plating conducted with an alloy layer, mainly consisting of iron, conducted on the steel sheet increases the cost of the material and affects chemical conversion as well as painting properties. Thus, application of such a plating on the entirety of a steel sheet for an automobile car body is not recommendable. Furthermore, since most of the rust preventatives or press oils which exhibit an excellent lubrication property cannot be readily removed, they may make the work in subsequent processes a troublesome one. The use of such a lubricant on some of the parts may be inhibited.
Thus, development of means for improving the press die sliding characteristics of the hot-dip zinc-coated steel sheet which eliminates problems involving the sliding characteristics during press forming when a normal rust-preventative or wash oil is used while restricting an increase in the material cost has been desired.
The sliding characteristics of the steel sheet with respect to the press die during press forming are affected by the properties and shape of the surface of the steel sheet as well as the lubrication effect obtained by using, for example, a press oil, a rust-preventative or a wash oil which is applied to the steel sheet. To improve the sliding property of the steel sheet, the general practice has been to utilize the lubrication effect of the liquid, such as a press oil or a rust-preventative, which is retained between the steel sheet and the press die by controlling the shape of the surface of the steel sheet. It is considered that in order to obtain the aforementioned lubrication effect, an increase in the average surface roughness (SRa) of the steel sheet is advantageous. However, excessive increases in SRa not only degrade the appearance of the steel sheet which has been subjected to coating but makes the lubricating effect nonuniform, deteriorating the press formability of the steel sheet or deforming the pressed part. Thus, an increase in the surface roughness alone of the steel sheet is not enough to obtain sufficient lubricating effect.